This invention relates to a partial stroke testing system and more particularly to a partial stroke testing system for the online testing of an emergency shut-off valve.
In the oil, gas, petroleum and power industries, various conditions may occur that necessitate immediate shut down or tripping of the operations, process or plant. In those industries, a majority of the final control element of shutdown systems are implemented with fast acting shut-off valves. In such industries, a majority of the shut-off valves remains open while the process is in a safe and controlled state. Such valves are closed only upon a plant trip rising from an out of control process or during a normal maintenance outage.
In practice, the testing of emergency shut-off valve is normally done during shut down of the process. However, there is tendency for such valves to stick or freeze due to corrosion or other reasons, which lead to an unsafe condition. This problem is exacerbated by economic conditions which have lead to a reduction in the frequency of shutdowns. For example, in some operations, a process may run continuously for one or more years without shutting down the process for maintenance.
State of the art emergency shutdown systems, which control the shut-off valves, have a number of features to detect plant or process failures and typically include redundancies for added reliability. However, such systems may not provide for the testing of shut-off valves itself other than stroking the valve. The problem is that full stroking or completely closing the valve causes an undesirable disruption in the process.
Recognizing that the emergency shut-off valve can be stroked partially as a safeguard against frozen or stuck valve has lead to a need for a simple, secure and reliable system for testing such valves without adversely effecting production. This approach will also improve the safety of the operation.
It is now believed that a partial stroke testing system in accordance with the present invention will provide a low cost, simple and reliable test for emergency shutdown valves in the oil, gas, petrochemical and power industries. Such tests will not adversely interrupt a process and will minimize or almost eliminate the risk of a xe2x80x9cfrozenxe2x80x9d emergency shutoff valve in the event of an out of control process. The system in accordance with the present invention is cost effective and has been designed to utilize a shut-off valve with fast acting piston actuator. The system is also applicable to slow acting valves.
In addition, the partial stroke testing system in accordance with the present invention will not interfere with a plant trip, i.e. a full shut down due to an emergency condition. Furthermore, if a partial stroke test is being conducted at the time of a plant trip, the partial stroke test will contribute to a more rapid closing of the emergency shut-off valve. Thus, the partial stroking design acts as a backup to the main trip mechanism. Accordingly, such systems should reduce the cost of insurance or risk coverage. In essence, the system partially closes the emergency shut-off valve to a predetermined position to test and confirm its ability to function and to ensure its availability on demand in the event of a plant emergency.
In essence, the present invention contemplates a partial stroke testing system or apparatus for the online testing of emergency shut-off valves. Such systems are particularly applicable in the oil, gas and petrochemical industries. The system is designed for implementation on an emergency shut-off valve, which is normally movable between a fully opened and a fully closed position. As contemplated by the preferred embodiment of the invention, the emergency shut-off valves will be in its fully opened position during operation of a process and will rapidly close to shut down the process in the event of an emergency. The system is designed for interfacing to the plant emergency shutdown system controller for generating electrical signals for initiating the test and to a source of pressurized gas such as compressed air for driving the system. The essential control components of an emergency shut-off valve are the main trip solenoid valve with manual reset, quick exhaust valve and the pneumatic actuator. On a trip signal, the solenoid valve de-energizes signaling the quick exhaust valve to vent the actuator and close the shut-off valve. The invention provides a test means for testing the emergency shut-off valve without fully closing the emergency shut-off valve in response to a signal from the plant emergency shutdown system controller. The test means includes a second solenoid valve for bleeding off compressed air to thereby move the emergency shut-off valve from a fully opened position to a partially closed position. Means such as closed limit switch which may be operable on the basis of the movement of the shut-off valve is provided. Means such as a timer set to a predetermined time limit to terminate the shut-off valve test in the event of the limit switch failure is also provided. In addition, means for initiating cyclical or periodical testing and reporting of test results of the partial stroking of the emergency shut-off valve is provided.
The invention will now be described in connection with the accompanying drawings.